1. Technical Field
The present invention relates to an electrostatic vibrator which is used as a reference frequency oscillator of various types of electronic apparatuses, and to an electronic apparatus having the electrostatic vibrator.
2. Background Art
In electronic apparatuses such as wireless portable apparatuses as represented by cellular phones, personal computers, or timepieces, there is an increasing demand to achieve size-reduction and higher accuracy. In such electronic apparatuses, a high-frequency signal source which is compact and stable is indispensable. An AT-cut quartz vibrator (hereinafter, simply abbreviated as an AT-cut) is a typical electronic component that meets this demand.
It is known that due to its good crystal stability, the sharpness of the resonance of the AT-cut, which is a quality index of an oscillation element, namely the Q value, is very large and exceeds 10000. This is why the AT-cut is widely used as a stable high-frequency signal source for wireless portable apparatuses and personal computers. However, it is also known that the AT-cut cannot sufficiently meet the strong demand in recent years for achieving size-reduction.
That is, with a dramatic improvement in silicon MEMS (Micro-Electro-Mechanical-System) techniques, almost every high-frequency electronic components other than the AT-cut are realized as one-chip components by being integrated with and bonded to an IC. However, since physical bonding of quartz monocrystals and silicon crystals is very difficult, the integration and bonding thereof is not possible. Therefore, the realization of the high-frequency electronic components including the AT-cut as one-chip components was not possible. This is the reason why the AT-cut cannot sufficiently meet the demand in recent years for achieving size-reduction.
In order to solve this problem, a vibrator that has been gathering attention in recent years is an electrostatic vibrator which uses silicon monocrystals and MEMS techniques. The electrostatic vibrator is a vibrator that converts a mechanical vibration having high Q characteristics of a vibrator formed of silicon to electrical signals by electrostatic force. Moreover, the electrostatic vibrator can realize impedance characteristics having high Q characteristics equivalent to a quartz vibrator. Furthermore, the electrostatic vibrator can realize integration and bonding with an IC which cannot be realized in the quartz vibrator as represented by the AT-cut. By this construction and corresponding functions, the electrostatic vibrator is a vibrator having extraordinary characteristics (see Non-Patent Literature 1, for example).
The electrostatic vibrator is manufactured by using an SOI wafer shown in FIG. 5. FIG. 5 is a schematic diagram of the SOI wafer. The SOI wafer is a three-layered wafer including a silicon substrate 501, a silicon oxide layer, namely a box layer 502, formed on the silicon substrate 501, and a silicon layer 503 formed on the box layer 502. An electrostatic vibrator according to the present invention is formed in the silicon layer 503 part shown in FIG. 5 using MEMS techniques such as DRIE.